Two transistor model is used to explain the principle of operation of the thyristor in a very simple way. Since a thyristor is a four-layered PNPN device, it can be considered as a combination of two transistors, one transistor as PNP and the other transistor as NPN. A two-transistor model is obtained by separating the two middle layers of the thyristor into two parts as shown in the below figure.
In this model, base current Ib1 of transistor Q1 is equal to the collector current Ic2 of transistor Q2 and vice-versa. The equivalent circuit of the two transistor analogy is shown below.
From the above equivalent circuit, the base and collector currents of the transistors Q1 and Q2 can be written as,Ib1 = Ic2 and Ic1 = Ib2 …(1)
Also, from the above figure, we have, cathode current is the sum of anode current and gate current.
We know that, collector current Ic is related to emitter current Ie as,
- α = Common-base current gain ≃ Ic/Ie
- Icbo = Leakage current of collector-base.
Hence, in the given equivalent circuit,
But, Ie1 of Q1 = Ia and Ie2 of Q2 = Ik. Subsituting the values of Ie1, Ie2 in equation 3, we get,
The sum of two collector currents is equal to an anode current. Therefore,
Substituting equation 4 in equation 5, we get,
From transistor analysis, we have,Ie1 = Ib1 + Ic1Ib1 = Ie1 – Ic1 …(7)
Substituting equation 2 in equation 6, we get,
If the leakage currents Icbo1 and Icbo2 of transistors Q1 and Q2 are negligibly small, then,
From the above equation, it can be analyzed that, if α1 + α1 = 1, then anode current Ia becomes infinity and hence thyristor enters into conduction state from non-conduction state. In other words, if Ig reaches a value such that α1 + α2 = 1, then SCR is turned ON and this action is called the Regenerative action.